In a typical cellular radio communications system (wireless communication system), an area is divided geographically into a number of coverage areas (namely, cells and cell sectors), each defined by a radio frequency (RF) radiation pattern from a respective base transceiver station (BTS) antenna. The base station antennae in the cells may then be coupled with a base station controller (BSC) (or radio network controller (RNC)), which may then be coupled with a telecommunications switch or gateway, such as a mobile switching center (MSC) or packet data serving node (PDSN) for instance. The switch or gateway may then be coupled with a transport network, such as the public switched telephone network (PSTN) or a packet-switched network (e.g., the Internet).
When an access terminal (such as a cellular telephone, pager, or appropriately equipped portable computer, for instance) is positioned in a coverage area, the access terminal communicates via an RF air interface with the BTS antenna of the coverage area. Consequently, a communication path can be established between the access terminal and the transport network, via the air interface, the BTS, the BSC and the switch or gateway.
As an access terminal moves between coverage areas of a wireless communication system, or when network conditions change or for other reasons, the access terminal may “hand off” from operating in one coverage area to operating in another coverage area. In a usual case, this handoff process is triggered by the access terminal monitoring the signal strength of signals that it is receiving in various available coverage areas, and the access terminal or the BSC determining when one or more threshold signal strength criteria are met. For instance, the access terminal may monitor signal strength in various available coverage areas and notify the BSC when a given coverage area has a signal strength that is sufficiently higher than the coverage area in which the access terminal is currently operating. The BSC may then direct the access terminal to hand off to that other coverage area.
In practice, communications over the air interface between a BTS (base station) and a access terminal will comply with a defined air interface protocol or “access technology.” Numerous such protocols are well known in the art, and others will be developed in the future. Examples of existing protocols include CDMA (e.g., 1xRTT, 1xEV-DO), iDEN, TDMA, AMPS, GSM, GPRS, UMTS, EDGE, WiMAX (e.g., IEEE 802.16), LTE, microwave, satellite, MMDS, Wi-Fi (e.g., IEEE 802.11), and Bluetooth. Each protocol may define its own procedures for handoff between coverage areas and may define other procedures and parameters related to air interface communication, such as particular air interface transmission mechanisms (e.g., coding, frequency, etc.)
Furthermore, in some cases, more than one air interface protocol might be implemented in a given market area. For example, a given market area might provide both legacy CDMA 1xRTT coverage under a standard protocol such as EIA/TIA/IS-2000 Rel. 0, Rel. A or another version thereof (hereafter “an IS-2000 protocol”) and also high data rate 1xEV-DO coverage according to a standard protocol such EIA/TIA/IS-856 Rel. 0, Rel. A, or another version thereof (hereafter “an IS-856 protocol”). In a system that provides two or more air interface protocols in a single area, a access terminal might not only hand off between coverage areas under a common air interface protocol (e.g., between IS-2000 sectors) but may also hand off between the different air interface protocols (e.g., between IS-2000 and IS-856) within a given physical location or when moving between locations. Handoff between different air interface protocols is known as “vertical” handoff.